1. Field of the Invention
The present invention relates to a head position demodulating method and disk apparatus for demodulating the head position by servo signals recorded on a disk, and more particularly to a head position demodulating method and disk apparatus for decreasing demodulated position errors due to the read characteristics of the head.
2. Description of the Related Art
A disk storage apparatus for recording and reproducing data on a rotating disk medium is widely used as a data storage apparatus. As FIG. 23 shows, the disk apparatus is comprised of a disk 94 for storing data, a spindle motor 96 for rotating the disk 94, a head 90 for recording and reproducing information on the disk 94, and an actuator 92 for moving the head 90 to the target position. Typical examples of this apparatus are a magnetic disk apparatus (HDD: Hard Disk Drive) and an optical disk apparatus (DVD-ROM, MO).
As FIG. 24 shows, on a magnetic disk apparatus, a plurality of position signals 100 for detecting the position of the head 90 are recorded in the arc position with respect to the rotation center 98 on the disk 94, and constitute a track. The position signal 100 is comprised of a servo mark, track number and offset information (servo information). The current position of the head 90 can be detected by using the track number and the offset information.
The difference between this position information and the target position is determined, drive amount is calculated according to the displacement amount (difference), and the drive amount for driving the actuator 92 is supplied, that is, current, in the case of a VCM (Voice Coil Motor) and voltage, in the case of an electro-strictive actuator, is supplied.
The servo signal (position signal) 100 on the disk 94 is either recorded by the disk apparatus itself, that is by the STW (Servo Track Write) method, or recorded by an external STW apparatus.
For this position signal, an area demodulating method using two-phase servo signals PosN and PosQ is used. FIG. 25 is a diagram depicting positional demodulating by the two-phase servo signals, and FIG. 26 is a diagram depicting two-phase servo signals thereof.
As FIG. 25 shows, the position signal (servo pattern) is comprised of four offset signals (servo bursts) A-D of which the phase is shifted from one another. In FIG. 25, the servo bursts A and B are recorded symmetrically with respect to the track position (dotted line position), and servo bursts C and D are recorded symmetrically with respect to the track boundary (solid line position).
From the output PosA−PosD when the head reads the servo bursts A-D, the two-phase servo signals PosN and PosQ are calculated by the following formula.PosN=PosA−PosBPosQ=PosC−PosDorPosN=(PosA−PosB)/(PosA+PosB)PosQ=(PosC−PosD)/(PosC+PosD)
As the detailed enlarged view in FIG. 26 shows, the signal of PosQ is placed with a ¼ track phase shift compared with PosN.
The demodulated position Pos is demodulated using either the absolute position of PosN or PosQ, whichever is smaller. In other words, as FIG. 25 shows, the acquired PosN or PosQ, of which the absolute value is smaller, indicated by the bold line in FIG. 25, is selected.
In this way, the amplitude of the reading output of each offset signal (PosA−PosD) from the head 90 is in proportion to the area of the offset signal (PosA−PosD) at the position of the head 90. In other words, the servo signal allows demodulating the position of the head by demodulating the area indicated by the amplitude.
By connecting the selected signals of the two-phase servo signals of the area demodulating method, the demodulated position of the actual position is acquired. The switching of PosN and PosQ occurs at this connection. It is preferable that the connected demodulated positions are a straight line, even with this switching.
As FIG. 27 shows, the causes of inhabiting the generation of a straight line are as follows; first cause is the deviation of the gain for converting the detected PosN and PosQ in track units (called position sensitivity). This gain changes depending on the detection sensitivity of the head, and if deviation occurs, the demodulated positions become different on the boundary between the section for demodulating PosN and the section for demodulating PosQ, where a step difference occurs.
Second cause is the fluctuation of the recording positions of the servo bursts A-D due to unstable writing during servo signal recording, and this is called RRO (Repeatable Run Out).
Third cause is that the read core width of the head is smaller than the track width, so PosN and PosQ are saturated by the head output, and by this a step difference occurs at the connected section.
To solve the problem of RRO and the problem of the measurement error of position sensitivity, the following methods are proposed.
(1) If the absolute value is |N|<|Q|, the position is demodulated by ±N/2 (|N|+|Q|), and if not, the position is demodulated by ±Q/2(|N|+|Q|) (U.S. Pat. No. 5,867,341, Official Gazette, “Disk drive system using multiple pairs of embedded servo bursts” (e.g. FIG. 6)).
(2) If the absolute value is |N|<|Q|, the position is demodulated by ±N/4|Q|, otherwise the position is demodulated by ±Q/4|N| (U.S. Pat. No. 6,369,974, Official Gazette, “Disk drive with method of constructing a continuous position signal and constrained method of linearizing such a position signal while maintaining continuity” (FIG. 9, FIG. 10)).
(3) If the absolute value is |N|<|Q|, the position is demodulated by ±N/√ (|N|^2+|Q|^2), and if not, the position is demodulated by ±Q/√ (|N|^2+|Q|^2) (Japanese Patent Application Laid-Open No. H9-198817, “Magnetic disk apparatus”).
All of these methods have a feature that (1) if PosN and PosQ are “0”, then the demodulated position is also 0, and (2), the boundary of the demodulated area of the PosN and PosQ has the same value for both cases of demodulating by PosN and demodulating by PosQ.
As the above formula shows, these methods need not measure position sensitivity, so the influence of a measurement error of position sensitivity can be avoided. Even if the write positions of the servo bursts of PosN and PosQ shift due to the influence of the write accuracy of the servo signals, a displacement does not occur at the boundary of the demodulated blocks of PosN and PosQ.
The demodulating formulas for PosN and PosQ are created such that the values match with the adjacent demodulation formula at the edge of the respective demodulated section. For example, when |N| and |Q| are the same in the method of U.S. Pat. No. 5,867,341, Official Gazette, “Disk drive system using multiple pairs of embedded servo bursts”, the values of the two formulas both become ±¼, which match each other.
These prior arts assume that the saturation of the signals PosN and PosQ, described in FIG. 27, is constant, and in the denominator of one signal, the signal component of the other signal is integrated, and by this, the step difference at switching is solved.
However, with the current demand for increased storage capacity, the track pitch must be narrower. Because of this, the read core width of the head decreases, which makes it difficult to manufacturer a head (especially an MR head) with a uniform detection characteristic. So the saturation width and the saturation area of PosN and PosQ, which are obtained by detecting the servo bursts, as shown in FIG. 28 (A), (B) and (C), change, depending on the detection performance of the read element.
Therefore in prior art, a deviation of the demodulated position tends to occur because of the change of the saturation width, since the change of the saturation width is not considered, even if the problems of position sensitivity and RRO are solved. In particular, this interferes with the improvement of positioning accuracy, which is currently demanded due to the decrease in track pitch.